In recent years, in addition to protection means against the front collision of a vehicle, an equipment has been made popular, which activates an occupant protection apparatus disposed on the lateral side of a vehicle where a collision occurs by detecting the acceleration generated in the lateral direction of the vehicle by using a side impact sensor (acceleration sensor). Besides, measures against an accident by the overturn of a vehicle (it is called “rollover”) also have been raised. Among rollover events of a vehicle, there occur a large number of accidents called “curb trip”: when a vehicle is driven at a high speed and cannot be sufficiently slowed down before a curve, the vehicle skids by the centrifugal force, and trips on an obstacle such as a curbstone to be led to overturn. The curb trip causes an acceleration G and a deceleration (velocity change) ΔV to be generated in the vehicle in a lateral direction (Y-axis direction) when the vehicle comes in contact with the obstacle. Thus, since the risk that the occupant can be thrown out of the vehicle is enhanced, it is required that the occupant protection system be activated as soon as possible.
The following related art is disclosed for measures against a curb trip:
An angular velocity sensor detecting the roll angular velocity of a vehicle is provided, the roll angle of the vehicle is calculated by integrating the roll angular velocity, the presence or absence of the possibility of the occurrence of the rollover of the vehicle is determined based on the value representative of the roll angle and the value representative of the roll angular velocity, and a determination is made based on the determination result whether or not the side occupant protection system mounted on the no-collision side of the vehicle is activated (see, e.g., Patent Document 1). However, in the related art, at an initial stage where the vehicle skids and the wheel on either of the right and left sides of the vehicle collides with a curbstone or the like, the generated lateral acceleration is detected with a side impact sensor, and the occupant protection system mounted on the collision-side is quickly activated. Further, the subsequent rollover is dealt with by using the angular velocity sensor as mentioned above.    Patent Document 1: JP-A-2004-256024
In the prior art activation apparatus for the occupant protection system providing measures against a curb trip is arranged as the above; however, there are the following problems.
When a side impact sensor is disposed on a member on the collision side of a vehicle so that the impact can be directly applied thereto upon a side impact, the impact is excessively inputted thereto in a collision mode where the side member is heavily deformed. At the same time, a comparatively large impact is inputted also with respect to a low-speed side-impact requiring no activation of the occupant protection system. Further, a large shock is inputted even when the door is strongly closed. For this reason, it is necessary that the threshold be held in a large value of a level such that the occupant protection system can be not accidentally activated.
By contrast, there is the following problem with respect to the case that the shock is delivered to the wheel portion of a vehicle such as curb trip: A large impact is not inputted to the side impact sensor disposed on the side member of the vehicle; thus, it is difficult to activate the occupant protection system at a proper timing even when an angle sensor or an angular acceleration sensor is used together therewith.
The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an activation apparatus for an occupant protection system enabling activation of an occupant protection system at an appropriate timing, even in the case where an impact is delivered to the wheel portion of a vehicle such as curb trip by setting a threshold with respect to the outputs from an acceleration sensor disposed on the side of a low-speed side-impact and/or a non-collision where a shock upon closing the door strongly is comparatively weak.